Integrated backside-illuminated imaging devices generally comprise light-detecting cells, or photosites, placed on the frontside of the substrate in which they are fabricated, and optical filters, for example color filters, on the backside of the substrate. Thus, light reaches the photosites directly after having passed through the color filters, in contrast to integrated frontside-illuminated devices in which the light passes through the back end of line (BEOL) interconnect layers before reaching the photosites.
These devices are generally produced in silicon-on-insulator (SOI) wafers. The photosites are then located in a silicon layer deposited on a silicon-dioxide (SiO2) layer forming a buried oxide (BOX). Color filters are placed neighboring the silicon-dioxide layer, the SOI wafer having been thinned so as to remove the silicon located under the buried oxide.
A silicon-nitride (Si3N4) layer may be deposited on the buried oxide after the SOI wafer has been thinned and before the color filters have been formed. This silicon-nitride layer forms, with the buried silicon-dioxide layer, an antireflection layer enabling light absorption to be maximized in the light-sensitive regions of the photosites. This being so, the antireflection layer formed by the buried oxide and the silicon-nitride layer is not effective enough for backside-illuminated imaging applications. This is because the thickness of the buried oxides, for example about 150 nanometers, does not allow sufficiently effective antireflection layers to be obtained.
Moreover, in order to isolate the photosites from one another deep isolating trenches are produced. These deep isolating trenches comprise a dielectric and extend from the frontside of the substrate in which the components are produced into the buried oxide layer. These trenches also allow optical waveguides to be formed, which waveguides enable light to be guided from the backside to light-sensitive regions of the imaging devices, thereby limiting crosstalk.
During fabrication of these isolating trenches, a step of etching is carried out from the frontside. This etching is stopped in the buried silicon-dioxide layer. This is because, in this etching step for forming a plurality of cavities, the cavities do not all have the same depth and a thick silicon-dioxide layer compensates for this etching non-uniformity. This being so, the use of a thick silicon-dioxide layer, for example about 150 nanometers in thickness, does not allow, as indicated above, an effective antireflection layer to be obtained.